tsan: fix mmap_lots test

If tsan runtime will try to allocate something during exit handling,
the allocation will fail because there is no VA whatsoever.
It's observed to fail with the following error in some cases:
failed to allocate 0x1000 (4096) bytes of DTLS_NextBlock.
So terminate the process immediately.

Reviewed-in: https://reviews.llvm.org/D96874